1. Field of the Invention
The present invention relates generally to audio amplifiers and drivers and specifically with the mitigation of audio pops.
2. Related Art
A significant issue with audio drivers in present technology arises from audio “pops.” Term “pop” in the audio field is an output disturbance caused by a sudden transition of a chip's power. In particular, the problem is especially pronounced when power is initially turned on (or equivalently off). Pops during when a chip powers up because the various components receive power and may cause voltage to be seen at the output even though there is no signal. Similarly, a pop can occur when the power is switched off because energy may be stored in the circuitry and the power may inadvertently be dissipated through the output even though no input signal is received. Furthermore, because of capacitive and inductive effects, the voltage can even spike when the power to a chip is switched on or off.
The pop issue manifests itself frequently in the driver or power portion of an audio circuit. This is commonly implemented with some form of amplifier. Two stage amplifiers are commonly used in audio applications. In particular the first stage is referred to as the amplifier stage and the second stage referred to as the output stage. Generally speaking, the amplifier stage supplies the gain and the output stage provides high current driving capability, low impedance. In the case of amplifier with differential inputs, the amplifier stage can supply either a single output representative of the difference between the input signals or can provide a differential output.
FIG. 1A illustrates a conventional design for a two stage amplifier. In this example, the circuit is operational amplifier 100 with amplifier stage 110 and output stage 160. Amplifier stage 110 comprises field effect transistors (FETs) 112, 114, 116, 118, and 120. A differential input VIN+ and VIN− are received by FET 116 and FET 118. An output based on the difference between VIN+ and VIN− is supplied to the output stage at node A. Output stage 160 comprises FET 162 which receives the output at node A and FET 164. The output stage produces an amplified output signal VOUT which is based on the difference between VIN+ and VIN−. The primary purpose of the output stage is not to provide gain but to maintain the output regardless of the current drawn through it. However, in some implementations the output stage may supply some amount of gain.
One of ordinary skill in the art would recognize there are countless designs for the amplifier stage and output stage. The design shown in FIG. 1A is a representative design. In order to simplify the remaining disclosure, where appropriate, the amplifier stage and/or output stage are represented by a symbol.
FIG. 1B illustrates a design for a general design for a two-stage amplifier. Amplifier 100 comprises amplifier stage 110 which derives a signal from VIN+ and VIN− and provides an output a node A. It also comprises output stage 160 which takes the output signal at node A and maintains output signal VOUT regardless of the current drawn by the attached load.
The difficulty with two-stage amplifiers is that generally they are inherently unstable. In order to address this issue, many compensation circuits exist. One of the most basic is to add an resistor and capacitor in feedback from the output to the input of the output stage.
FIG. 2 illustrates a two-stage amplifier with a basic compensation network. In amplifier 200, capacitor 202 and resistor 204 are added to output stage 220 in feedback from the output to input at node A. The original circuitry described as output stage 160 in amplifier 100 are now referred to as the core output stage to avoid confusion. This feedback from the output to node A provides stability to the two-stage amplifier. However, it provides another source of pop. For example, when amplifier stage 110 is powered up the voltage at node A may be spike. Through capacitor 202 and resistor 204, that spike can be transmitted to the output causing the pop.
Previous solutions have been applied to audio systems having additional circuitry. Specifically, FIG. 3 illustrates an audio system which comprises in addition to amplifier 200, a sound output apparatus comprising low pass filter 302 and output circuit 304 is inserted between VOUT and the load such as a speaker. In many applications low pass filters are used prior to attaching an audio system to a load. In addition output circuit 304 comprises an electrostatic protection circuit which is used to shunt harmful external static electricity away from the remainder of the audio system.
In such an implementation, previous solutions have added anti-pop circuit 310 into the sound output apparatus. Anti-pop circuit 310 comprises shunting capacitor 314 and switch 312. Control circuit 316 closes switch 312 before power is switched on and switched off. When switch 312 is closed, the output of low pass filter 302 is shunted to ground through shunting capacitor 314, thus draining any voltage spikes to ground before they can manifest themselves as a peak.
The primary drawback to this type of solution is that it requires a sound output apparatus to be placed external to the amplifier. In modern audio systems, there is a desired to eliminate the sound output apparatus. In particular, because the low pass filter is placed near the output, the low pass filter must be designed to accommodate high power. As a result, the low pass filter is bulky, expensive and consumes a lot of power. The elimination of the low pass filter and/or output circuit can reduce power consumption and expense, but it also eliminates the opportunity to deploy an anti-pop circuit such as anti-pop circuit 310.
Thus there is a need in the industry for an inexpensive, compact solution that reduces or eliminates the audio pop in an audio amplifier without the need for expensive additional circuitry.